Organic light emitting devices (OLEDs) make use of thin film materials which emit light when excited by electric current. Presently, the most favored organic emissive structure is referred to as the double heterostructure (DH) OLED, shown in FIG. 1A. In this device, a substrate layer of glass 10 is coated by a thin layer of a transparent, conductive oxide such as indium-tin-oxide (ITO) 11. Next, a thin (100-1000 .ANG.) organic hole transporting layer (HTL) 12 is deposited on ITO layer 11. Deposited on the surface of HTL 12 is a thin (typically, 50 .ANG.-1000 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-1000 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
Often, the EL 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency and lifetime of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and a thick, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17" is opaque. When proper bias voltage is applied between top electrode 17 and contacts 15 and 16, light emission occurs from emissive layer 13 through the glass substrate 10.
Another known organic emissive structure referred to as a single heterostructure (SH) is shown in FIG. 1B. The difference between this structure and the DH structure is that multifunctional layer 13' serves as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13' must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
Yet another known LED device is shown in FIG. 1C, illustrating a typical cross sectional view of a single layer (polymer) OLED. As shown, the device includes a glass substrate 1 coated by a thin ITO layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg, Ca, Li or other conventionally used low work function metal.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in U.S. Pat. No. 5,294,870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light. Fluorescent media are positioned between the blue OLED and the substrate in certain parts of the pixels. The fluorescent media absorb light emitted by the blue OLED and emit red and green light in different regions of the same pixel.
In general, light emission brightness in conventional OLED technology can be somewhat limited. The ability to achieve higher brightnesses would expand the number of potential applications for this technology. Additionally, in conventional OLED light emitting devices there is a loss of light that is waveguided within device layers, such as the substrate or OLED layers. There thus exists the need for a high brightness OLED light emitting device in which the problem of lost waveguided light is minimized.